1. Field of the Invention
The present invention relates to transmission line transformers, and particularly, to transformers for use in a magnetic field.
2. Description of the Prior Art
A particular type of transmission line transformer is known as the balun (balanced-to-unbalanced). A balun is used to match unbalanced lines to balanced lines. For example, a typical 300 ohm twin lead line (balanced) used in television antenna systems requires a balun to couple the line to a 75 ohm coaxial cable (unbalanced). Baluns may be 1:1 for matching 50 ohm lines to 50 ohm loads or some other ratio in accordance with a given system. A dipole antenna requires a balun for coupling to a coaxial cable. Often baluns use ferrite cores to enhance their operation. Baluns may be linear employing ferrite rod cores or torroidal employing ferrite ring cores. Reference is made to a more detailed discussion of baluns in "Transmission Line Transformers" by Jerry Sevick,
Chapter 8, the American Radio Relay League, Newington, Conn. 1987; "The ARRL Antenna Handbook," American Radio Relay League, 1974, pages 115, 116 and Proceedings RF Expo East, Nov. 10-12, 1986, Boston, Mass., "Practical Wideband RF Power Transformers, Combiners, and Splitters" by R. K. Blocksome, pages 1-21.
In NMR (Nuclear Magnetic Resonance) systems, antennas are frequently used to receive rf signals generated in a static magnetic Bo field. Certain of these antennas are referred to as surface coils and take a variety of shapes and configurations. One type of such a coil is a birdcage coil comprising a pair of spaced rings coupled by a set of parallel linear rods. The coil includes spaced capacitors for tuning the coil to a given rf frequency, typically about 64 MHz. Such coils may further include means for operating the coils linearly or in quadrature, the latter arrangement increasing the signal-to-noise ratio in a known manner.
In an NMR system a body coil which creates the Bo field is a large birdcage which receives the body of a patient being examined. The body coils are usually of limited dimensions for receiving a patient table with the patient lying supine or prone on the table. Typically such body coils are about 20-24 inches (50-60 cm) in internal diameter. That leaves about 12-14 inches (30-35 cm) above the table for the smaller surface or birdcage coil for examining a particular part of the body. The smaller birdcage coil may be used locally to examine a persons head, for example. The head is inside the coil while in the Bo field and coaxial cables connect the local coil to the imaging system outside the field. Such cables in some cases may be connected to a balun before connection to instrumentation. The balun is sometimes referred to as a trap. The balun is used to prevent rf currents that may be induced on the cable ground shield in the rf field generated by a body, e.g., the head, in response to the system generated rf flux. Such currents need be prevented because they degrade the efficiency of the local coil and because only a small amount of power is available from body tissue for sensing by the local coil. Typically all of this power would go toward producing currents in the local coil. These currents are amplified and sent on to the rest of the NMR system. Any currents induced in the cable shield represent a loss of power and must be eliminated.
In one case the balun is the coaxial cable coiled into a transformer and mounted adjacent to the birdcage coil. This is not entirely satisfactory because a considerable length of cable is present between the balun and local birdcage coil. In other cases, the local birdcage coil is constructed with a flat copper sheet material end in place of one of the rings. The coaxial cables are connected to the coil via this copper sheet which serves as a shield between the cables and the coil.
To further avoid generating unwanted induced currents in the coaxial cabling, the cables may be connected to the local birdcage coil normal to the coil longitudinal axis and to the Bo field for a distance of about the birdcage diameter, typically about 8 inches (20 cm). However, this is usually not practical due to the limited amount of space available within the body coil as discussed above. Additionally, forming the coaxial cable into a balun is also not practical in certain implementations because the cables limit the balun diameter to too large a dimension for use in the limited space due to the limited bend radius of the cable.
In the present implementation, it is desired to use the birdcage coil in quadrature for examining the knee. This requires both ends of the coil to be open to permit the leg to pass through. This prohibits the use of the copper shield. The use of cables normal to the cage is also prohibited in this case because of lack of room in the body coil. The use of the balun arrangement by coiling the coaxial cable in a region next to the birdcage coil is also not satisfactory for the reasons given.